Automatic dishwashing detergents constitute a generally recognized distinct class of detergent compositions whose purpose can include to break down and remove food soils; to inhibit foaming; to promote the wetting of wash articles in order to reduce or eliminate visually observable spotting and filming; to remove stains such as might be caused by beverages such as coffee and tea or by vegetable soils such as carotenoid soils; to prevent a buildup of soil films on washware surfaces; and to reduce or eliminate tarnishing of flatware without substantially etching or corroding or otherwise damaging the surface of glasses or dishes. The problem of glassware corroding during washing the cycle of an automatic dishwashing appliance has long been known. Current opinion is that the problem of corrosion in glassware is the result of two separate phenomena. On one hand, the high pH needed for cleaning causes silica hydrolysis. This dissolved silica/silicate, together with silicates added purposely to prevent china and metal corrosion, deposit on the glass surface leading to iridescence and clouding. On the other hand, builder removal of chelate metal ions from the glass surface, and the subsequent metal ion leaching that follows renders a less durable and chemical resistant glass. After several washes in an automatic dishwashing appliance, both phenomena can cause damage to glassware such as cloudiness, scratches, and streaks.
Most consumers agree that corrosion of glassware from use of detergent compositions in automatic dishwashing (ADW) is one of their most serious unmet needs. ADW detergent compositions containing zinc or magnesium salts of organic acids for improved protection against glass corrosion are known. As these salts are sparingly soluble, they are used for controlled release of reactive zinc species. The performance of soluble zinc salts in detergent compositions is difficult to control as precipitates of insoluble zinc salts with other ions in the wash liquor will occur. Yet insoluble zinc salt precipitates may deposit on both the glassware and on the ADW appliance elements itself. Furthermore, some insoluble zinc salts may be too inert to deliver the needed Zn2+ ions, as for example zinc oxide (ZnO). Aluminum sulfate salts have also shown promise, but formulatabilty issues remain. For example, flocculation with a polymer thickener and a slight negative on oxygen bleach performance requires an encapsulation approach, which can add formulation costs. Rinse aids containing zinc or magnesium salts are also known but are used by only a small number of consumers, therefore, it is desirable to be able to deliver Zn2+ ions through-the-wash. Thus, there is a continuing need to develop alternative automatic dishwashing detergent compositions containing Zn2+ ions that provide the abovementioned benefits yet reduce the problem of glassware surface corrosion experienced in through-the-wash applications.